New modem, no registration?

This seems baffling. I'm on Comcast Cable in Denver, and my SB5120 modem (owned, not rented) got fried by a nearby lightning hit. I replaced it with a Linksys CM100 (which outwardly looks like the exact same unit with different plastic around it and a smaller wall wart).

When I turned the new modem on, it simply connected, got its IP address, and was off and running. No modem registration page, no error message...No machine on my system has ever had any Comcast-supplied software on it, and I haven't communicated the modem's MAC address or S/N to them. So how do they know where my packets are coming from? Or do they?

I'm running Tomato 1.17 on a WRT54GL; does Tomato maybe have some way of automating modem registration? Thanks...

I know nothing of Comcast, nor of CM100s, but if your router is controlling the modem, it's your router that does the connecting. The modem only supplies the modulation necessary to talk the language of the hardware line (DSL, cable, whatever), but your router's MAC, etc. are at play.